Fermentation and antimicrobial characteristics of Lactobacillus plantarum and Candida tropicalis from Nigerian fermented maize (akamu)

Obinna-Echem ,Kuri ,Jane Beal

doi:10.7455/ijfs/3.2.2014.a5

Abstract

This study investigated the ability of Lactobacillus plantarum strains (NGL5 and NGL7) and Candida tropicalis (NGY1) previously identified from akamu -a Nigerian fermented maize food with probiotic L. plantarum LpTx and Saccharomyces boulardii SB20 to ferment ground maize slurries based on pH, acidity, microbial biomass, levels of sugars and organic acids, and their antimicrobial activity against Salmonella enterica serovar Enteritidis NCTC 5188, Escherichia coli NCTC 11560, Bacillus cereus NCIMB 11925, Staphylococcus aureus NCTC 3750 and Listeria monocytogenes NCTC 7973 using an agar spot assay. L. plantarum strains either as single or mixed starter cultures with the yeasts had growth rates ≥0.15 h -1 ,with pH significantly (p≤0.05) decreased to ≤3.93 after 12 h and then to ≤3.52 after 72 h and lactic acid >84 mmol L -1 . The yeasts had growth rates ≥0.18 h -1 but pH was ≥4.57 with lactic acid levels ≤20.23 mmol L -1 after 72 h in the single culture fermentation. There was no inhibition in modified MRS agar: 0.2% glucose and 0.2% glucose without Tween 80. Inhibition halos in MRS agar varied from 10.6 to 23.9 mm. S. bourladii was more inhibitory towards L. monocytogenes (8.6 mm) and B. cereus (5.4 mm ) than was C. tropicalis (1.1 and 3.3 mm for L. monocytogenes NCTC 7973 and B. cereus NCIMB 11925 respectively) (0.9 mm) in malt extract agar. This study showed that C. tropicalis was less inhibitory to the pathogens while antimicrobial activities of the L. plantarum strains were mainly due to acidity and the L. plantarum strains either as single or mixed cultures with the yeasts demonstrated strong fermentation ability, with significant decrease in pH which is vital in the choice of starter for product safety.

Highlights

Fermentation is one of the oldest food preparation methods considered as safe and acceptable for improving the quality and safety of foods
The pH of the L. plantarum single (≤3.43) or mixed culture fermentation with the yeasts (≤3.52) were such that would not permit the survival of most unwanted microorganisms, while the decrease in pH of the yeast single culture fermentation was ≥4.57
This was observed with the L. plantarum strains, either as a single or mixed starter cultures with the yeasts

Summary

Introduction

Fermentation is one of the oldest food preparation methods considered as safe and acceptable for improving the quality and safety of foods. Spontaneous fermentation of cereal-based foods is borne out of competitive activities of endogenous or contaminating microorganisms and its initiation may take 24 - 48 h. It is important that the selected microbial inoculant to facilitate the process should be adaptive to the substrate from the onset of fermentation so as to prevent the growth of unwanted microorganisms to a level that would impair the product’s quality. Lactic acid bacteria (LAB) and yeast occur as part of the natural microbial population in spontaneously fermented foods and as starter cultures in the food and beverage industry (Shetty & Jespersen, 2006). In a recent study by ObinnaEchem, Kuri, and Beal (2014), the LAB population of a selected Nigerian traditional fermented maize food called akamu was found to be dominated by strains of Lactobacillus plantarum, L. fermentum, L. delbrueckii subsp. The presence of LAB and yeasts have been associated with many other fermented maize doughs and porridges

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